Satellite cellular communications systems function similarly to terrestrial cellular systems in that they require the location of the user to be known in order to authorize access or, in some cases, perform a system registration procedure for the user. Terrestrial cellular systems typically use groups of cells referred to as location areas to determine acceptability of a user (see, for example, The GSM System for Mobile Communications, Mouly and Pautet (1992)). The user, upon turning on the telephone, is sensed by the cell that he is in, and the cell base station reports the user as active to a base station controller (BSC) which is controlling a group of cells that denotes a particular location area. In turn the base station controller notifies a Mobile Switching Center (MSC) that the user is in the location area. The MSC then operates to determine the acceptability of the user to access the system from the current location area. Upon changing location areas, the user automatically reinitiates these reporting activities, thus allowing roaming access between location areas. Typically, the terrestrial cellular system operates in a frequency band that is not shared with other services. As such, the need to protect portions of the service area from user terminal generated interference does not arise.
The process is similar in a satellite communication system except that the location areas are much larger, and in the case of a low earth orbit (LEO) satellite system, are constructed by the gateway to satellite to user terminal co-visibility contours generated by the motion of the satellites, and the location of the gateway on the surface of the earth. In the case of many satellite systems, and unlike terrestrial cellular systems, the frequencies are shared with other radio frequency-using services, such as Radio Astronomy, Microwave Landing Services, Radio Location, and other services operating on the ITU basis of a Primary or Secondary Service. The satellite service may also be a Primary or Secondary Service and thus needs to coordinate with the other Primary service or services, or the satellite service may be a Secondary Service and be required to avoid interference. Most of the time the coordination process and avoidance of interference leads to the specification of a certain protection region within the service area (the service area is also called a location area). Generally, these protection areas are small with respect to the service area itself.
The problem that arises can thus be stated as follows: how will the satellite communication system allow access from the service area, while rejecting users from within a specified distance of or from within an arbitrarily defined protection region within the service area, based upon information received by the Mobile Switching Center from the user terminal or from signals emanating from the user terminal?
Various prior art approaches are known in the fields of position location, the generation of service and location areas by LEO satellite systems, as well as registration and access to cellular systems.
Of particular interest are: “Resolution of Position Location in Low Earth Orbit Systems”, a paper presented at IMSI, 1993, “Euteltracs”; U.S. Pat. No. 4,972,456, Kaczmarek et al.; U.S. Pat. No. 5,844,521, Stephens et al.; U.S. Pat. No. 5,920,284, Victor; U.S. Pat. No. 5,974,356, Doyal et al.; U.S. Pat. No. 5,126,748, Ames et al; U.S. Pat. No. 5,017,926, Ames et al; U.S. Pat. No. 5,946,618, Agre et al.; commonly assigned U.S. Pat. No. 6,072,768, Wiedeman, et al.; and commonly assigned U.S. Pat. No. 5,303,286, Wiedeman.